UV Technology

How UV Curing Works

In its most basic form, UV curing involves a photo-chemical reaction which converts a liquid or semi-liquid organic compound to a hard plastic-like polymer. The heart of this reaction is a special compound, known as a “photoinitiator”, which absorbs light and then uses the absorbed light energy to initiate and propagate the curing reaction.

Unlike conventional drying processes which use heat to evaporate water or solvents from a material, UV curing involves a total conversion of liquid material to a solid state. This lack of solvents makes UV curing an attractive alternative in cases where solvent emissions must be reduced. The hard plastic-like cured polymer has superior physical properties (such as abrasion-resistance, gloss, and chemical resistance). These properties are used effectively in many printing and industrial applications.

The UV curing process is very fast, usually completed in fractions of a second. This means reduced space requirements and a decrease in Work In Process. Racking or secondary drying operations are eliminated with UV curing. In many cases, quality can be improved through the elimination of ink transfer, or surface blemishes caused by contact between a partially dry ink or coating and another surface.

Any UV curing process requires a UV light source and an ink or coating specially formulated for the UV curing process. Both the light source and the UV curable material are normally designed for each specific application.

UV Light Energy

UV energy is commonly referred to as “light” because it conforms to the optical rules of visible light. In the electromagnetic spectrum, UV is located in the higher frequency range:

Electromagnetic Energy Use Frequency - Hertz (cycles per second)
Direct Current 0
Electrical Power 50 - 60
AM Broadcast Radio 550,000 - 1,600,000
FM Broadcast Radio 88,000,000 - 108,000,000
Microwave Oven 2,450,000,000
IR Shortwave Dryer 300,000,000,000,000
Sunlight Peak (Yellow) 536,000,000,000,000
Ultra Violet Black Light 750,000,000,000,000

Light energy is composed of individual particles known as “photons”. Planck’s law describes the energy level of a photon of a particular frequency as:

E = hv

Where E = energy of the photon, h =Planck’s constant, and v = frequency

As frequency increases, a point is reached in the electromagnetic spectrum where photons have enough energy to power photochemical reactions. This is what gives UV light its commercial usefulness. There are a number of organic compounds which absorb in the UV region and which have the chemical capabilities to use this energy to promote a photochemical reaction. The most common photochemical reaction is photosynthesis – where green plants absorb visible light photons from the sun and convert carbon dioxide and water to carbohydrates.

For convenience sake, the upper end of the electromagnetic spectrum is specified by wavelength. Infra Red is specified in units of wavelength called “microns” ( 10-6 or one millionth of a meter). Visible and UV are specified in “nanometers” ( 10-9 or one billionth of a meter).

The UV region is divided into four parts. VUV, or Vacuum UV, has a wavelength of 100 to 200 nanometers (nm). These wavelengths are not useful in UV curing, because atmospheric gases ( notably Oxygen and Nitrogen ) strongly absorb these wavelengths. UVC (200 – 280 nm) has the most energetic of the wavelengths used in UV curing. Photons in the UVC are important for surface cure and promote surface properties such as hardness, stain resistance, and abrasion resistance. Photons in the UVB (280 – 320 nm) contribute to bulk cure and photons in the UVA (320-400 nm) promote through cure, especially with thicker film layers.

Visible light has also found commercial use in curing. Light in the 400-420 nm region (violet) has been used to cure white pigmented coatings. Light in the 440-460 region (blue) has been used in dental offices for tooth repair.

The choice of the right output spectrum is very important for successful curing applications. Not only must the lamp output match the absorption spectrum of the photoinitiator, but the effects of pigments and other additives must be taken into consideration. In general terms, the thicker or more heavily pigmented the UV curable layer is, the longer the wavelength should be. This is because longer wavelengths tend to penetrate deeper.

There are three commonly used lamp spectra for UV curing. The most common is the Mercury spectrum, also known as the “H” spectrum. This is produced by using only Mercury as the fill material of the lamp. The Mercury spectrum output has a series of peaks distributed throughout the UV spectrum and is used as a general purpose lamp. Most printing applications use the Mercury spectrum. Strong output in the UVC region makes the Mercury spectrum the lamp of choice where surface cure properties are very important. An example of this is UV curing on vinyl flooring.

When other additives are mixed with the Mercury fill, the output spectrum of the lamp changes dramatically. The “D” spectrum is formed when Iron halides are mixed with the Mercury. This spectrum has most of its output in the UVA region. The Iron lamp is used in applications which require curing of a thick layer of UV material.

When Gallium halides are mixed with the Mercury, the Gallium ( “V” ) spectrum is produced. This lamp has a characteristic purple hue, due to the location of most of its output in the violet region (400 – 425 nm) of the visible spectrum. It is used to cure thick white pigmented coatings. This is because the lamp output very closely matches a cure window in the coating formed by the absorption of the photoinitiator and the transmission curve of the white Titanium Dioxide pigment.

For spectral output charts of these lamps, click the “UV Spectra” button on the navigation bar to the left.

Other metal halides or combinations of metal halides are used for special applications. An example is Indium halide which has a strong output peak in the 440 – 460 nm (blue) region. This spectrum is used in dental offices to cure coatings on teeth.

UV Curing Materials

The photochemistry involved in UV curable materials is very complicated and far beyond the scope of this introduction. The chemistry of the UV curable material is tailored to the specific process with the method of application, UV source, and desired properties of the cured material all factored in.

UV curable materials consist of a minimum of three elements. These are the photoinitiator, oligomers, and monomers. The photoinitiator’s role is to absorb the UV photon and transfer this energy to the curing process. Oligomers act as the backbone of the cured material to help give it the desired physical properties and the monomers create the cross linking action which cures the material.

In addition to these elements, pigments may be added to achieve desired color or opacity and other additives may be used to control uncured properties such as tack or viscosity.

UV ink and coating companies have specialized in various applications with no one company serving all applications. If you contact us, we will be happy to recommend a supplier for your application.

UV Curing Safety

UV Chemistry

Like any industrial chemical, un-cured UV Materials should be treated with care. Refer to the MSDS (material safety data sheets) provided by the UV-material supplier for specific guidance on the actual UV material being used. Many UV materials can irritate the skin and mucous membranes (eyes and nose). Always wear latex gloves when handling UV material and wash any residue from the skin with soap and water. Material left on the skin for a period of time can cause an itching sensation and ultimately a rash. Keep hands away from eyes, nose, and mouth. Wash hands thoroughly before smoking, eating, or going to the bathroom. Once cured, the UV material can be handled normally without any concerns.

UV Light

Although UV light occurs naturally, especially on a sunny day, the intensity of UV light emitted by a UV lamp is much greater than found outdoors. UV lamp system manufacturers routinely incorporate shielding to protect the operator from this high intensity UV light. A properly designed light shield should prevent the operator, when in a normal position, from looking directly at the bulb or at the cure zone (where the focused light from the lamp strikes the substrate being cured). Secondary reflections contain a lower level of UV light (due to the poor UV reflectivity of most materials), but they still need to be controlled. As a general rule of thumb, light that is uncomfortable to the eye should not escape from the lightshield. A glow of light from a light shield is considered a safe level. In some cases, it is not feasible to reach this level of light attenuation. In those instances, proper protection of eyes and skin is necessary. This includes UV blocking glasses, gloves, long sleeve shirts, and possibly face protection. Never look directly at the bulb or cure zone.

UV light has well-known effects on unprotected skin and eyes. Sunburn is caused when the skin absorbs too much UV light. The unshielded light from a UV lamp can cause the same kind of burns. Snow blindness or “welder’s eye” occurs when too much UV light is absorbed by the outer surface of the cornea. Direct light from a UV lamp can also cause this condition. UV burns to the skin and cornea, though normally painful, completely heal in a few days time.

Ozone

Ozone is a type of oxygen molecule, created when a normal 2 atom oxygen molecule is broken apart by high energy and then recombines into the 3 atom ozone molecule. In nature, UV light from the sun and the electrical discharge of a lightning bolt both create ozone.

Oxygen absorbs UV strongly in the 185nm wavelength. During lamp start up, the cold quartz tube transmits 185nm UV. As the quartz heats up to operating temperature, the transmission curve of the quartz shifts towards the longer wavelengths and 185nm light is no longer able to pass through the quartz. For this reason, ozone is generated by UV lamps mostly during start up.

In the concentrations found in UV curing systems, ozone can possibly irritate the mucous membranes of the eyes, nose, and throat. Most UV systems are designed to discharge exhaust air outside the plant to eliminate this possibility.

Ozone is very unstable, and rapidly decomposes to oxygen in the presence of UV light moisture, metals, organic materials and heat. Since these materials are prevalent in the UV curing process, ozone produced by the lamps quickly decomposes.